The instant invention is in the field of Advanced Glycation End-products (AGEs), their formation, detection, identification, inhibition, and inhibitors thereof.
Protein Aging and Advanced Glycosylation End-products
Nonenzymatic glycation by glucose and other reducing sugars is an important post-translational modification of proteins that has been increasingly implicated in diverse pathologies. Irreversible nonenzymatic glycation and crosslinking through a slow, glucose-induced process may mediate many of the complications associated with diabetes. Chronic hyperglycemia associated with diabetes can cause chronic tissue damage which can lead to complications such as retinopathy, nephropathy, and atherosclerotic disease. (Cohen and Ziyadeh, 1996, J. Amer. Soc. Nephrol. 7:183-190). It has been shown that the resulting chronic tissue damage associated with long-term diabetes mellitus arise in part from in situ immune complex formation by accumulated immunoglobulins and/or antigens bound to long-lived structural proteins that have undergone Advanced Glycosylation End-product (AGE) formation, via non-enzymatic glycosylation (Brownlee et al., 1983, J. Exp. Med. 158:1739-1744). The primary protein target is thought to be extra-cellular matrix associated collagen. Nonenzymatic glycation of proteins, lipids, and nucleic acids may play an important role in the natural processes of aging. Recently protein glycation has been associated with xcex2-amyloid deposits and formation of neurofibrillary tangles in Alzheimer disease, and possibly other neurodegenerative diseases involving amyloidosis (Colaco and Harrington, 1994, NeuroReport 5:859-861). Glycated proteins have also been shown to be toxic, antigenic, and capable of triggering cellular injury responses after uptake by specific cellular receptors (see for example, Vlassara, Bucala and Striker, 1994, Lab. Invest. 70:138-151; Vlassara et al., 1994, PNAS(USA) 91:11704-11708; Daniels and Hauser, 1992, Diabetes 41:1415-1421; Brownlee, 1994, Diabetes 43:836-841; Cohen et al., 1994, Kidney Int. 45:1673-1679; Brett et al., 1993, Am. J. Path. 143:1699-1712; and Yan et al., 1994, PNAS (USA) 91:7787-7791).
The appearance of brown pigments during the cooking of food is a universally recognized phenomenon, the chemistry of which was first described by Maillard in 1912, and which has subsequently led to research into the concept of protein aging. It is known that stored and heat-treated foods undergo nonenzymatic browning that is characterized by crosslinked proteins which decreases their bioavailibility. It was found that this Maillard reaction occurred in vivo as well, when it was found that a glucose was attached via an Amadori rearrangement to the amino-terminal of the xcex1-chain of hemoglobin.
The instant disclosure teaches previously unknown, and unpredicted mechanism of formation of post-Amadori advanced glycation end products (Maillard products; AGEs) and methods for identifying and characterizing effective inhibitors of post-Amadori AGE formation. The instant disclosure demonstrates the unique isolation and kinetic characterization of a reactive protein intermediate competent in forming post-Amadori AGEs, and for the first time teaching methods which allow for the specific elucidation and rapid quantitative kinetic study of xe2x80x9clatexe2x80x9d stages of the protein glycation reaction.
In contrast to such xe2x80x9clatexe2x80x9d AGE formation, the xe2x80x9cearlyxe2x80x9d steps of the glycation reaction have been relatively well characterized and identified for several proteins (Harding, 1985, Adv. Protein Chem. 37:248-334; Monnier and Baynes eds., 1989, The Maillard Reaction in Aging, Diabetes, and Nutrition (Alan R. Liss, New York); Finot et al., 1990, eds. The Maillard Reaction in Food Processing, Human Nutrition and Physiology (Birkhauser Verlag, Basel)). Glycation reactions are known to be initiated by reversible Schiff-base (aldimine or ketimine) addition reactions with lysine side-chain xcex5-amino and terminal xcex1-amino groups, followed by essentially irreversible Amadori rearrangements to yield ketoamine products e.g. 1-amino-1-deoxy-ketoses from the reaction of aldoses (Baynes et al., 1989, in The Maillard Reaction in Aging, Diabetes, and Nutrition, ed. Monnier and Baynes, (Alan R. Liss, New York, pp 43-67). Typically, sugars initially react in their open-chain (not the predominant pyranose and furanose structures) aldehydo or keto forms with lysine side chain xcex5-amino and terminal xcex1-amino groups through reversible Schiff base condensation (Scheme I). The resulting aldimine or ketimine products then undergo Amadori rearrangements to give ketoamine Amadori products, i.e. 1-amino-1-deoxy-ketoses from the reaction of aldoses (Means and Chang, 1982, Diabetes 31, Suppl. 3:1-4; Harding, 1985, Adv. Protein Chem. 37:248-334). These Amadori products then undergo, over a period of weeks and months, slow and irreversible Maillard xe2x80x9cbrowningxe2x80x9d reactions, forming fluorescent and other products via rearrangement, dehydration, oxidative fragmentation, and cross-linking reactions. These post-Amadori reactions, (slow Maillard xe2x80x9cbrowningxe2x80x9d reactions), lead to poorly characterized Advanced Glycation End-products (AGEs).
As with Amadori and other glycation intermediaries, free glucose itself can undergo oxidative reactions that lead to the production of peroxide and highly reactive fragments like the dicarbonyls glyoxal and glycoaldehyde. Thus the elucidation of the mechanism of formation of a variety of AGEs has been extremely complex since most in vitro studies have been carried out at extremely high sugar concentrations.
In contrast to the relatively well characterized formation of these xe2x80x9cearlyxe2x80x9d products, there has been a clear lack of understanding of the mechanisms of forming the xe2x80x9clatexe2x80x9d Maillard products produced in post-Amadori reactions, because of their heterogeneity, long reaction times, and complexity. The lack of detailed information about the chemistry of the xe2x80x9clatexe2x80x9d Maillard reaction stimulated research to identify fluorescent AGE chromophores derived from the reaction of glucose with amino groups of polypeptides. One such chromophore, 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole (FFI) was identified after nonenzymatic browning of bovine serum albumin and polylysine with glucose, and postulated to be representative of the chromophore present in the intact polypeptides. (Pongor et al., 1984, PNAS(USA) 81:2684-2688). Later studies established FFI to be an artifact formed during acid hydrolysis for analysis.
A series of U.S. Patents have issued in the area of inhibition of protein glycosylation and cross-linking of protein sugar amines based upon the premise that the mechanism of such glycosylation and cross-linking occurs via saturated glycosylation and subsequent cross-linking of protein sugar amines via a single basic, and repeating reaction. These patents include U.S. Pat. Nos. 4,665,192; 5,017,696; 4,758,853; 4,908,446; 4,983,604; 5,140,048; 5,130,337; 5,262,152; 5,130,324; 5,272,165; 5,221,683; 5,258,381; 5,106,877; 5,128,360; 5,100,919; 5,254,593; 5,137,916; 5,272,176; 5,175,192; 5,218,001; 5,238,963; 5,358,960; 5,318,982; and 5,334,617. (All U.S. Patents cited are hereby incorporated by reference in their entirety).
The focus of these U.S. Patents, are a method for inhibition of AGE formation focused on the carbonyl moiety of the early glycosylation Amadori product, and in particular the most effective inhibition demonstrated teaches the use of exogenously administered aminoguanidine. The effectiveness of aminoguanidine as an inhibitor of AGE formation is currently being tested in clinical trials.
Inhibition of AGE formation has utility in the areas of, for example, food spoilage, animal protein aging, and personal hygiene such as combating the browning of teeth. Some notable, though quantitatively minor, advanced glycation end-products are pentosidine and Nxcex5-carboxymethyllysine (Sell and Monnier, 1989, J. Biol. Chem. 264:21597-21602; Ahmed et al., 1986, J. Biol. Chem. 261:4889-4894).
The Amadori intermediary product and subsequent post-Amadori AGE formation, as taught by the instant invention, is not fully inhibited by reaction with aminoguanidine. Thus, the formation of post-Amadori AGEs as taught by the instant disclosure occurs via an important and unique reaction pathway that has not been previously shown, or even previously been possible to demonstrate in isolation. It is a highly desirable goal to have an efficient and effective method for identifying and characterizing effective post-Amadori AGE inhibitors of this xe2x80x9clatexe2x80x9d reaction. By providing efficient screening methods and model systems, combinatorial chemistry can be employed to screen candidate compounds effectively, and thereby greatly reducing time, cost, and effort in the eventual validation of inhibitor compounds. It would be very useful to have in vivo methods for modeling and studying the effects of post-Amadori AGE formation which would then allow for the efficient characterization of effective inhibitors.
Inhibitory compounds that are biodegradable and/or naturally metabolized are more desirable for use as therapeutics than highly reactive compounds which may have toxic side effects, such as aminoguanidine.
In accordance with the present invention, a stable post-Amadori advanced glycation end-product (AGE) precursor has been identified which can then be used to rapidly complete the post-Amadori conversion into post-Amadori AGES. This stable product is a presumed sugar saturated Amadori/Schiff base product produced by the further reaction of the early stage protein/sugar Amadori product with more sugar. In a preferred embodiment, this post-Amadori/Schiff base intermediary has been generated by the reaction of target protein with ribose sugar.
The instant invention provides for a method of generating stable protein-sugar AGE formation intermediary precursors via a novel method of high sugar inhibition. In a preferred embodiment the sugar used is ribose.
The instant invention provides for a method for identifying an effective inhibitor of the formation of late Maillard products comprising: generating stable protein-sugar post-Amadori advanced glycation end-product intermediates by incubating a protein with sugar at a sufficient concentration and for sufficient length of time to generate stable post-Amadori AGE intermediates; contacting said stable protein-sugar post-Amadori advanced glycation end-product intermediates with an inhibitor candidate; identifying effective inhibition by monitoring the formation of post-Amadori AGEs after release of the stable protein-sugar post-Amadori advanced glycation end-product intermediates from sugar induced equilibrium. Appropriate sugars include, and are not limited to ribose, lyxose, xylose, arabinose, glucose, fructose, maltose, lactose, mannose, fructose and galactose. In a preferred embodiment the sugar used is ribose.
The instant invention teaches that an effective inhibitor of post-Amadori AGE formation via xe2x80x9clatexe2x80x9d reactions can be identified and characterized by the ability to inhibit the formation of post-Amadori AGE endproducts in an assay comprising; generating stable protein-sugar post-Amadori advanced glycation end-product intermediates by incubating a protein with sugar at a sufficient concentration and for sufficient length of time to generate stable post-Amadori AGE intermediates; contacting said stable protein-sugar post-Amadori advanced glycation end-product intermediates with an inhibitor candidate; identifying effective inhibition by monitoring the formation of post-Amadori AGES after release of the stable protein-sugar post-Amadori advanced glycation end-product intermediates from sugar induced equilibrium. In a preferred embodiment the assay uses ribose.
Thus the methods of the instant invention allow for the rapid screening of candidate post-Amadori AGE formation inhibitors for effectiveness, greatly reducing the cost and amount of work required for the development of effective small molecule inhibitors of post-Amadori AGE formation. The instant invention teaches that effective inhibitors of post-Amadori xe2x80x9clatexe2x80x9d reactions of AGE formation include derivatives of vitamin B6 and vitamin B1, in the preferred embodiment the specific species being pyridoxamine and thiamine pyrophosphate.
The instant invention teaches new methods for rapidly inducing diabetes like pathologies in rats comprising administering ribose to the subject animal. Further provided for is the use of identified inhibitors pyridoxamine and thiamine pyrophosphate in vivo to inhibit post-Amadori AGE induced pathologies.